In some conventional steering assisting systems, an electric power steering apparatus or a variable transfer ratio steering apparatus is employed to assist a steering operation of a steering wheel in a vehicle. In other conventional steering systems both the electric power steering apparatus and the variable transfer ratio steering apparatus are employed.
The electric power steering apparatus generates torque for assisting the steering operation of the steering wheel. The variable transfer ratio steering apparatus variably controls a transfer ratio of rotation between a steering angle of the steering wheel and a steered angle of steered wheels (tire wheels).
The electric power steering apparatus and the variable transfer ratio steering apparatus stop respective steering assisting operations, when a predetermined failure occurs therein. For instance, the torque generation operation or the variable transfer ratio control operation may be stopped. The steering operation of the steering wheel by a vehicle driver will not be so influenced by the operation stop of the variable transfer ratio steering apparatus. However, it will be influenced by the operation stop of the electric power steering apparatus, because the steering operation will not be power-assisted.
JP 2003-40123A and U.S. Pat. No. 6,820,715 (JP 2003-200840A) propose improved electric power steering apparatuses, which continue power-assisting operations even when a failure arises at some parts.
In JP 2003-40123A, for instance, a pair of bridge circuits are provided in a motor drive circuit and a pair of coil sets are provided in a brushless motor. Thus, even when a failure occurs in one bridge circuit or one coil set, the other bridge circuit or the other coil set is used in place of the failing bridge circuit or the coil set to continue the power-assisted steering operation. In U.S. Pat. No. 6,820,715, three electric motors operable in parallel and three control circuits operable in parallel are provided. Thus, even when a failure occurs in one motor or one control circuit, another motor or another control circuit is used to continue the power-assisted steering operation. These electric power steering apparatuses need additional set of parts, which cause complicated configuration and high cost.